This application is based on French Patent Application No. 01 11 046 filed Aug. 23, 2001, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. xc2xa7119.
1. Field of the Invention
The present invention relates to the technology of fiber optic transmission and in particular to an optical modulator which improves transmission quality and thereby improves detection of signals transmitted in the presence of noise.
2. Description of the Prior Art
Telecommunication networks have expanded considerably over the last several decades. Although most calls, whether carrying voice (telephone calls) or data, have long used standard telephone lines, which have a low bit rate, the formidable expansion of the Internet and all other data networks, whether in the public or private sector, since the middle eighties has led to an enormous demand for bandwidth. To face up to this exponential increase in the quantity of information to be transported, and which relates to all types of media, i.e. as much to voice as to data, such as electronic mail (E-mail), text and picture file transfer, video distribution and, most importantly, the massive use of the Internet and the World Wide Web (WWW), new technologies have had to be developed, as transmission over electrical media (metal lines, copper) has proved to be too limited in performance over long distances.
At least insofar as the core of these networks is concerned, transmission is now mostly via optical fibers at very high bit rates. The rate of exchange of data, or information bits, is routinely measured in gigabits per second. This means that one billion (109) bits can be exchanged every second over a 1 Gbit/s line. In practice international standards exist to standardize transmission and to ensure the interworking of equipment. The most widespread of these standards is the SONET (Synchronous Optical NETwork) standard. The SONET standard is primarily a North American standard, and its European counterpart is the SDH (Synchronous Digital Hierarchy) standard. These standards are for the most part mutually compatible and standardize transmission speeds of 2.48 Gbit/s (SONET OC-48), 10 Gbit/s (SONET OC-192) and 40 Gbit/s (SONET OC-768).
Although communication equipment now communicates via a network of optical fibers, which can be very extensive and cover a city or a country, and can include intercontinental transmission, and carries pulses of light generally obtained from a coherent light emitter (laser), it remains the case that the communication equipment itself is still essentially based on electrical technologies and the peripheral circuits that constitute the equipment must be capable of being interfaced efficiently and at low cost to the devices sending and receiving light signals interfaced to the optical fibers.
In its simplest form, binary signals conveying the information bits referred to above are simply transmitted by modulating at two levels the light emitter, usually a laser. Thus the optical signal is generated at two power levels and the laser is switched from a level at which it emits a sufficient quantity of light to be received by the optical receiver situated at the other end of the fiber to a level at which it does not emit any or much light, in which state it must be considered to be turned off. The receiver is thus in a position to discriminate the two levels corresponding to an information bit (a xe2x80x981xe2x80x99 generally corresponding to the state in which the laser is emitting light, although the opposite convention is obviously equally feasible). If the emitter continues to emit between two consecutive xe2x80x981xe2x80x99 and returns to the off state only to transmit a xe2x80x980xe2x80x99, the modulation mode is known as non return to zero (NRZ) modulation. It is cheap and well suited to the mode of operation of lasers, which are turned on or off to transmit each bit of information.
However, with the considerable increase in transmission speeds required for the new generation of communication equipment (up to 40 Gbit/s) it becomes more and more difficult to control turning a laser on or off for a short time and to ensure that the receiver detects correctly the information transmitted, in particular in the presence of noise, which in practice is always present.
Thus the object of the invention is to provide an improved optical modulator which, whilst retaining the simplicity of the NRZ method, makes a significant improvement to it through the possibility of improved detection of the signals transmitted, i.e. by achieving a better signal to noise ratio than the conventional NRZ method.
The invention therefore provides an optical modulator for transmitting data encoded in NRZ mode by modulating the intensity of a light signal, the modulator including means for shaping the light signal including shaping means for producing a signal of low bandwidth from an input control signal, and a laser modulator having a transfer function favoring the extinction time of the resulting light signal.
According to an important aspect of the invention, the ratio of the transition time of the resulting light signal from a minimum luminous intensity level to a maximum luminous intensity level to the bit time of the signal is from 0.1 to 0.6, and the ratio of the holding time at the maximum intensity level to the total bit time less the transition time between the minimum luminous intensity level and the maximum luminous intensity level is from 0.1 to 0.85.
The aims and objects and the characteristics and advantages of the invention emerge better from the following detailed description of a preferred embodiment of the invention, which is illustrated by the accompanying drawings.